Exact measurements

| November 30, 2006

Knowing how to use accurate measuring tools will set you apart from the amateurs.

There are many things on a truck you can’t do by eye or feel. One of the biggest differences between an amateur mechanic and a competent technician is knowing how to use special tools that precisely measure important values.

Even a beginning do-it-yourself technician should know how to use a torque wrench because using one is essential to properly torque wheel lug nuts. Learning to use more complex tools will help you if you decide to become an owner-operator. Many independent truckers graduate to doing major work on their vehicles to save money by taking the time to learn proper procedures, including special tool use.

We visited Pennco Technical Institute in Blackwood, N.J., where instructors Bill Brandt and Nelson Barr showed us the details of using special tools.

Using a torque wrench
The word “torque” means twisting force. Torque is equivalent to weight when measured a certain distance from the center of a shaft, bolt or screw. “Pounds-feet,” often called “foot pounds,” are the specified number of pounds measured one foot from the center of a shaft or bolt. “Inch-pound” just means torque measured on a different scale – only one inch from the shaft’s center. For example, 120 inch-pounds would be equivalent to 10 pounds-feet.

Torque wrenches are extremely important because one of the secrets of good mechanical work is tightening something with just the right amount of force. Fasteners need to be tight enough to keep parts from bending or flexing in operation, which can cause them to loosen. But too tight is just as bad as not tight enough, because screws and bolts stretch and lose strength when tensioned past a certain point.

In a few cases, where such fasteners must be used only once, the bolt is often stretched just to the point of maximum strength. Once this has been done, it is permanently altered or “fatigued” and must be replaced when the part is removed and then reassembled. In both cases, creating exactly the right torque or twisting force is critical if the assembly is to hold together.

This inch-pound torque wrench (A) is used to check torque when assembling smaller parts. It can use various sizes of sockets with the proper extensions or adapters. You position the back pointer to zero, and then apply the torque by twisting the wrench. After you release the torque, the black pointer will remain at the point of maximum torque to record what you did.

This more traditional design (B) is for larger bolts and higher torques. When you apply torque, the shaft rotates or deflects slightly while the pointer stays straight and indicates the torque on the scale in either pounds-feet or Newton-meters.

A third type for larger bolts and torque ratings is a click type. This type wrench is used to torque cylinder heads and wheels. It is set by turning the sleeve until its edge lines up with the number of pounds-feet desired. You install the proper size socket, place the socket securely over the bolt, and turn with gradually increasing force. It’s important to make sure to hold the socket on tight with your other hand so it won’t slip off and damage the bolthead, Brandt says. The wrench will click and release slightly when reaching the rating.

After use, always make sure to turn the sleeve back until the torque is zero to unload the spring. This prevents fatigue of the spring, which would alter the calibration.

Calibration is a way to check that a torque wrench is still accurate with a known-accurate wrench. Study the instructions that come with the tool to see how often it needs to be calibrated.

Using a micrometer
Often known as a “mic,” (pronounced “mike”), a micrometer measures very short distances like clearances between close-fitting parts with great precision. Mics take some education to read because of the way they are constructed. To get the final reading, you need to read several scales and add them together. This relates to the fact that we use a decimal number system with the numbers in each position representing 1/10 the distance represented by the number to its left.

First, fit the mic around the object to be measured (C). Turn the main thimble (like a knob) to make the object just fit inside the mic’s opening; then tighten the instrument with the smaller, ratcheting thimble. This works like a torque wrench and unlocks, keeping you from tightening too much. Once the ratchet works, rotate the locking tab to retain the reading and remove the object being measured.

Then read the three scales this way:

  1. Read the large numbered line to the left of the edge of the thimble, which represents tenths of an inch. In this case, the number is 0.100, or one tenth of an inch.

  2. Count the additional lines between the number above and the edge of the thimble. Each of these lines represents 1/4 of the above, or 0.025 inch, which is 25 thousandths of an inch. There are three lines visible. Multiply this number (3) by 0.025, which = 0.075, or 75 thousandths.
  3. Look at the lines printed around the thimble itself. Read them at the “reading line,” which is the line at the center of the horizontal scale on the shaft of the mic. These lines each represent 0.001 inches (1 thousandth of an inch). If the fourth line lines up with the reading line, that = 0.004.
  4. Add the three numbers mathematically. You just have to line up the decimal points directly above one another. 0.100 + 0.75 = 0.175 + 0.004 = 0.179 inch.

Using a ball gauge
A ball gauge is a device that measures “inside diameter,” or how large an opening is. When rebuilding engines, it is often necessary to measure inside diameters, for example, to read the wear in a valve guide. The valve guide is a sleeve in the cylinder head in which the valve slides up and down as it opens and closes.

Here, ball gauges of various sizes are carefully fitted into a valve guide (D). The ball must fit easily into the guide – don’t force it. But there should be a very slight drag, indicating that it is touching the metal all around. The ball gauge is then measured by the mic as done above to get the final dimension.

Using a dial indicator
Dial indicators measure the distance a part slides or will move under force. An ideal use for dial indicator is a turbocharger shaft’s “end play.” End play is the distance the shaft can be slid forward and backward inside the turbo housing. When the shaft’s bushings wear out, end play becomes too great. Checking this is a great way to find whether or not a turbo needs replacement.

First remove the hose connecting the air cleaner to the turbo compressor inlet. Then push the turbo shaft all the way to the rear – away from you.

Mount the dial indicator onto the turbo housing in a position that will cause the stem of the indicator to contact the end of the shaft straight on. Once in position, depress or rotate the calibration knob to bring the pointer to zero. Then gently pull the turbo shaft toward you as far as it will go. The scale will now show the reading in thousandths of an inch (E). You count the lines as with the dial indicator. The engine manual will give a tolerance such as 0.003-0.005. For example, if the reading were, say, halfway between 0.004 and 0.005 on the scale, the turbo would be OK. If at 0.006, it would have to be replaced.

Using a snap gauge
A snap gauge is used to measure large inside diameters, like that of a cylinder liner after miles of wear. The gauge is sprung apart like a toilet paper roll mounting shaft.

  1. Hold the gauge together to make sure it does not come apart, and then unlock it by turning the thumbwheel counterclockwise.

  2. Press the parts together, hold and then turn the thumbwheel to tighten.
  3. Have the liner sitting on a level surface. Insert the gauge into the opening in the top of the liner and position it so it is as level as possible in the center of the opening and the shaft is as vertical as possible. Then loosen the thumbwheel at the top of the shaft so the gauge will snap open against the liner walls (F).
  4. Tighten the thumbwheel and then remove the gauge. Then measure its length with a large micrometer designed for this purpose to get the actual dimension (G).

Using feeler gauges
Setting an engine’s overheads is a complex job. You need to turn the engine to several different positions and set the valves in an exact sequence. The correct information is in a factory engine manual.

A feeler gauge is used to measure a small clearance like between a valve and the rocker lever that opens and closes it. You get the clearance out of the manual, say 0.003 inch, and then pick out a gauge marked with that thickness. The position of the rocker is adjusted with a slotted screw held in place after the work is done by a locknut.

  1. Loosen the locknut and back off the adjusting screw.

  2. Insert the feeler gauge flat between the valve keeper and the rocker lever (H).
  3. Slowly slide the gauge back and forth while gently tightening the locking screw. The clearance is correct when the feeler gauge is just touching both parts, which gives a slight resistance.
  4. Tighten the locknut while holding the adjusting screw in position with a screwdriver. Then recheck the adjustment by sliding the gauge between the two parts.

Some engines allow a tolerance. Detroit Diesels use a stepped “go-no go” gauge (I). If the thinner part slips in between the rocker and the valve but the thicker part won’t fit, the adjustment is within tolerance.

Injector and rack setting tools
When you become more proficient, you will also be able to set injector height, which is much like setting a valve, and several other settings that are critical on older diesels. This will be done using special tools designed for only one type of engine. This is a set of tools used to set injector height, and a mechanical device called a rack that controls the injectors on pre-electronic Detroit Diesels (J). Making settings such as these is part of setting the overheads and helps maintain peak engine performance and smoke-free operation.

Injector height is set by installing a height gauge’s prong into a hole in the cylinder head right next to the injector. The injector rocker is then adjusted up and down just as was done with the valves, using an adjusting screw. A flag or horizontal part on the gauge is then turned toward the injector. When the height is right, the flag just fits across the top of the injector spring retainer.

Learning to use tools such as these will enable you to take the guesswork out of working on your truck, and may even allow you to successfully overhaul a major component some day.


For More Information:
Pennco Technical Institute
(856) 232-0310
www.Penncotech.com

Fluke Corp.
(800) 443-5853
Us.fluke.com/usen/home

KD Tools
(800) 688-8949
www.kd-tools.com

MATCO Tools
(866) 289-8665
www.matcotools.com

Comments are closed.

OverdriveOnline.com strives to maintain an open forum for reader opinions. Click here to read our comment policy.